The mitochondrial electron transport (or respiratory) chain is a series of enzyme complexes in the mitochondrial membrane that is responsible for the transport of electrons from NADH to oxygen and the coupling of this oxidation to the synthesis of ATP (oxidative phosphorylation). ATP then provides the primary source of energy for driving a cell's many energy-requiring reactions.
ATP synthase (F.sub.1 F.sub.0 ATPase) is the enzyme complex at the terminus of this chain and serves as a reversible coupling device that interconverts the energies of an electrochemical proton gradient across the mitochondrial membrane into either the synthesis or hydrolysis of ATP. This gradient is produced by other enzymes of the respiratory chain in the course of electron transport from NADH to oxygen. When the cell's energy demands are high, electron transport from NADH to oxygen generates an electrochemical gradient across the mitochondrial membrane. Proton translocation from the outer to the inner side of the membrane drives the synthesis of ATP. Under conditions of low energy requirements and when there is an excess of ATP present, this electrochemical gradient is reversed and ATP synthase hydrolyzes ATP. The energy of hydrolysis is used to pump protons out of the mitochondrial matrix.
ATP synthase is, therefore, a dual complex. F.sub.0 is a transmembrane proton carrier or pump, and F.sub.1 is the catalytic portion that synthesizes or hydrolyzes ATP. The mammalian ATP synthase complex from bovine heart mitochondria consists of sixteen different polypeptides (Walker, J. E. and Collinson, T. R. (1994) FEBS Lett. 346: 39-43). Six of these polypeptides (subunits .alpha., .beta., .gamma., .delta., .epsilon., and an ATPase inhibitor protein, IF.sub.1) comprise the globular catalytic F.sub.1 portion of the complex, which lies outside of the mitochondrial membrane. The remaining ten polypeptides (subunits a, b, c, d, e, f, g, F6, OSCP, and A6L) comprise the proton-translocating, membrane spanning F.sub.0 portion of the complex. A stalk portion of 40-50 angstroms in length connects the F.sub.1 and F.sub.0 portions together and serves to transfer electrochemical energy between them. Subunits from both portions of the ATP synthase complex contribute to the stalk; the .gamma., .delta., and .epsilon. subunits from F.sub.1, and F6, OSCP, b, and d subunits from F.sub.0.
F6 (coupling factor 6) is important for ATP-Pi exchange and for the oligomycin-sensitive ATPase activity in the complex. F6 is a small 76 amino acid coupling factor that, like most mitochondrial proteins, is the product of a nuclear gene that is imported into the mitochondria. F6 is synthesized as a 108 amino acid precursor; the N-terminal 32 amino acids constitute an import signal peptide that targets the protein to the mitochondrial membrane (Higuti, T. et al. (1990) Biochem. Biophys. Res. Commun. 171(3):1079-86). Similar import sequences are found in the F6 precursor proteins for rat, cow, and human (Higuti, et al. supra; Walker, J. E. et al. (1987) Biochemistry 26:8613-19; Higuti, T. et al. (1991) Biochem. Biophys. Res. Commun. 178(2):793-99). The key characteristics of this import signal peptide are a net basic charge with no acidic amino acids and an amphipathic .alpha.-helical region with opposing positively charged and hydrophobic faces that are important for transporting the protein through the mitochondrial inner membrane (Higuti et al. (1990), supra).
Transcriptional regulation of these nuclear encoded genes appears to be the predominant means for controlling the biogenesis of ATP synthase. Defects and altered expression of ATP synthase and other enzymes in the respiratory chain are associated with a variety of disease conditions, including neurodegenerative diseases, myopathies, and cancer.
The discovery of polynucleotides encoding a human mitochondrial F6 subunit and the molecules themselves provides a means to investigate the control of cellular respiration under normal and disease conditions, and satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the treatment or prevention of cancer, myopathies, and neurodegenerative diseases.